Semiconductor devices for converting incident thermal radiation into an electrical potential are known in the art. These devices include a semiconductor P/N junction structure supported on a substrate. Thermal radiation focused on the P/N semiconductor junction results in photon energy being absorbed by an electron in a semiconductor valence band, creating a mobile electron in the conduction band and a mobile hole in the valence band. The diffusion of the mobile charge carriers interacts with the intrinsic electric field at the P/N junction separating positive and negative charge across the junction. Electrical potential is generated across the junction proportional to the incident thermal radiation intensity.
The use of such thermovoltaic semiconductors requires a thermal radiation wavelength which generates charge carriers in a P/N semiconductor junction. For a radiant source temperature less than 1500.degree. K, the majority of thermal radiation energy is below the semiconductor band gap and unusable. To prevent the parasitic absorption of unusable energy, it is desirable to reflect energy which is not converted into electrical potential back to the source where it may be reabsorbed.
To avoid the generation of parasitic absorption by unusable radiant energy, various filtering techniques have been employed to filter out the unusable portion of the thermal radiation spectrum preventing it from reaching the P/N junction. Interference filters which have a sharp cut-off wavelength characteristic to provide a high reflection for unusable radiation have been employed for this purpose. These interference filters, however, have a relatively short effective stop band wavelength over which the radiation is reflected. Thus, above a certain wavelength, the filter becomes ineffective in blocking the higher wavelength portion of the unusable thermal radiation from reaching the P/N junction, thus, resulting in parasitic absorption of unusable energy which cannot be converted to an electrical potential.
The present invention is directed to reducing the parasitic absorption of this unusable energy which has heretofore not been completely reflected by state of the art interference filters.